-A 2D ultrasound is a black and white image that is produced by waves. It is used to screen for fetal abnormalities and assess fetal well-being.
-A 3D ultrasound uses the same sound technology as 2D, but adds the third element which is depth. This also gives you the image, but produces more detail. The 3D allows you to have a "sneak peak" at what your baby will look like.
-A 4D ultrasound (also referred to as "live 3D") expands on the concept of a 3D ultrasound, but rather than having a still image you can actually see movement. 4D is the latest in ultrasound technology.

When is a good time for 3D/4D ultrasound?

Studies have found the optimal time to do a 3D ultrasound is between 27 to 31 weeks. During this time period, babies are building more fat or are "filling in". This makes the facial features appear more as they are at birth rather than having a "skeletal" look. At this time, there is a still adequate amount of fluid around the baby. However, as the baby grows, it gets increasingly crowded. After 32 weeks, it is difficult to get a good 3D/4D image due to fetal size and the fact that the amount of amniotic fluid has decreased.

What factors determine the quality of the image?

The quality of the image depends upon the baby's position, size, movement and amniotic fluid. The most important factor is the position of the baby. In order to get a good 3D/4D image, you must have a good 2D image without any obstruction. At times, the baby may put an arm in front of the face or have part of the face resting against the wall of the uterus, which can limit the quality of the image. Other critical factors are the amount of amniotic fluid (the more the better), fetal activity, and the amount of tissue between the ultrasound probe and the baby.